1. Field of the Invention
This invention relates to a process for the preparation of powdered tungsten metal, and more particularly to a fluidized bed process for producing tungsten metal of controlled particle size from particles of tungsten oxide.
2. Background of the Disclosure
Tungsten and its alloys are widely used in high temperature structural elements, including lamp filaments, other electrical elements, and electrical instruments. Tungsten is also used as an alloying element with other metals in the manufacture of structural parts in which strength, ability to withstand high temperatures, and oxidation and corrosion resistance, particularly at high temperatures, are required. Tungsten is also a starting material for the manufacture of tungsten carbide, which is very hard and is used in tools, such as machining, cutting and well drilling tools and other applications where hardness is required.
Tungsten metal powder is the starting material of choice for all the above products. The tungsten metal powder should be highly pure and preferably of substantially uniform particle size. High purity is readily obtained by present methods, but uniformity of particle size is not always achieved. Wide particle size distribution complicates fabrication into many goods and products. Most tungsten metal powder is produced commercially in a two step process in which (1) ammonium paratungstate (APT) is converted to a tungsten oxide in particle form in a rotary tube furnace, and in which (2) the oxide particles, contained in rectangular boats, are reduced to tungsten metal powder in a stationary tube furnace. The tungsten oxide particles obtained in the first step are chemically non-uniform and are of varying particle size, even with carefully controlled furnace conditions. Tungsten metal powder obtained in the second step is likewise not uniform in particle size. This is due to variations in the starting oxide, in temperature and relative hydrogen/water contents within the particle bed. It is not possible to obtain a uniform product with narrow particle size distribution according to this method. A uniform product with broad particle size distribution is achieved only by blending various lots of powder, since particle size distribution of the tungsten metal powder as produced varies from lot to lot.
Another commercial process for preparing tungsten uses a rotary process for converting APT to tungsten metal in a single step. This process requires continuous and accurate control of APT feed rate. Product quality is also dependent upon APT feed rates. Throughput is related to the desired particle size. Throughput of a typical rotary furnace can vary from 20 kilograms per hour for fine particles to 70-80 kilograms per hour for coarse particles. In practice, the single step rotary reduction requires approximately the same degree of process control as the first process and is slightly less costly. On the other hand, particle size distribution is generally even broader than that in the first mentioned process.
Other processes have been reported in the literature but have not gained wide acceptance. For example, U.S. Pat. No. 3,324,007 describes a fluidized bed process for obtaining tungsten metal powder from tungsten hexafluoride.
Early attempts to prepare tungsten metal powder by fluid bed techniques resulted in very broad (and hence less desirable) particle size distribution than those obtained in stationary and rotary furnaces.